Question

How many distinct positive integers are possible with the digits 1, 3, 5, 7 without repitition ?

Answer 1

Possible number of,

single-digit numbers = $4$

two-digit numbers = $4\times 3=12$

three-digit numbers = $4\times 3\times 2=24$

four-digit numbers = $4\times 3\times 2\times 1=24$

Thus, total number of distinct positive integers without repitition = $4+12+24+24=64$.